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Challenges Faced By India In The Design Of Pressurised Heavy Water Nuclear Power Plants

Article Introduction

For a nuclear power to be cost-effective, conscious efforts on the part of all of us to meet our commitments to project time schedules and costs are required.

Article Description

The nuclear power industry has been developing and improving reactor technology for almost five decades and is preparing for the next generations of reactors to fill orders expected in the next five to twenty years. Several generations of reactors are commonly distinguished. Generation I reactors were developed in 1950-60s and outside the UK none are still running today. Generation II reactors are typified by the present US fleet and most in operation elsewhere. Generation III are the Advanced Reactors discussed in this paper. The first are in operation in Japan and others are under construction or ready to be ordered. Generation IV designs are still on the drawing board and will not be operational before 2020 at the earliest.

Reactors derived from designs originally developed for naval use generate about 85% of the worlds nuclear electricity. These and other second-generation nuclear power units have been found to be safe and reliable, but they are being superseded by better designs. A three-stage programme was drawn up to develop nuclear power in India to match with our unique resource position of limited Uranium and large Thorium reserves. For optimum use of our Uranium resources, the first stage of this programme is based on construction of Pressurised Heavy Water Reactors (PHWRs). Indigenously developed Pressurised Heavy Water Reactor technology for 220 MWe units has been a commercial success. Eight such units are in operation and four more are in final stages of construction and commissioning. Total capability for design, construction and operation of these plants has been successfully demonstrated.

Based on this experience, Nuclear Power Corporation has launched construction of two 500 MWe PHWR units at Tarapur - Tarapur Atomic Power Project, Units 3&4 (TAPP 3&4) in October 1998. Some of the technical challenges, which have been successfully overcome in setting up 220 and 500 MWe PHWR plants, are summarized in this article. Electricity is an essential part of an industrialized society, forming one of the pillars of quality of life today. Electricity is the speed for development of a country. Thermal power generation, a major contributor to electricity production in India, turns 100 this year. About 50 years ago, when we gained our independence, India barely had a total installed capacity of 1300 MWe. This has since been enhanced to over 85,000 MWe today. Thermal power stations are the major contributors in this mammoth achievement. Even then, our per capita energy availability remains much below the world average. It is a challenge for all of us in the electricity generation industry to join forces to secure reliable electric supply on a long-term basis.

Visionary architects of science and technology of modern India foresaw the imperative need to develop all the necessary technologies for power generation. It was a result of this recognition that a three-stage programme was drawn up to develop nuclear power in India to match our unique resource position of limited Uranium and large Thorium reserves. For optimum use of the available Uranium resources, the first stage of this programme is based on Pressurised Heavy Water Reactors (PHWRs). These reactors not only use natural Uranium efficiently but also provide Plutonium as a by-product. The Plutonium recovered from the spent fuel will facilitate use of our large Thorium reserves for power production in subsequent stages of the programme. Pressurised Heavy Water Reactor technology developed in our country for 220 MWe units has been a commercial success. Eight such units are in operation and four more are in final stages of construction and g. Total capability for design, construction and operation of these plants has been successfully demonstrated. Based on this experience, Nuclear Power Corporation has launched construction of two 500 MWe PHWR units at Tarapur - Tarapur Power Project, Units 3&4 (TAPP 3&4) in October 1998.

General description of 500 MWe PHWR
A PHWR is fuelled with natural uranium dioxide fuel and is moderated and cooled by heavy water. Separate circuits are used for the moderator and coolant. While the moderator is at low temperature and pressure, the coolant is normally maintained at high temperature and pressure. The reactor vessel, known as calandria, is made of austenitic stainless steel and is located horizontally in a shielded, water filled, concrete vault with stainless steel lining. It houses 392 pressure tubes, also called coolant tubes, made of Zirconium 2.5% Niobium. Zirconium, like heavy water, absorbs only a negligible amount of neutrons. Each pressure tube is surrounded by a thin zircaloy calandria tube. The annular space between the calandria tube and coolant tube is filled with carbon dioxide gas, providing an insulating gap between the coolant and moderator. Each pressure tube contains a string of fuel bundles, each about half metre long. Because a PHWR uses natural uranium, the fuel needs to be replenished on a daily basis. An important feature of the PHWR is that it is refuelled while on power, thus avoiding frequent shutdown of the reactor for refuelling. The fuel bundles are inserted at one end of pressure tube and spent fuel bundles from the channel are discharged at the other end. Bi-directional fuelling in alternate pressure tubes prevents the reactor from having all fresh fuel bundles at one end and irradiated fuel bundles at the other thus resulting in a more symmetrical neutron flux shape. Also, fuelling is normally carried out along the direction of coolant flow.

The closed loop primary coolant circuit (also known as primary heat transport system) has four mushroom type steam generators, four PHT pump motor units, a pressuriser and connected headers and feeders which are further connected to the 392 coolant tubes containing in all 5096 fuel bundles. The PHT system also incorporates a feed and bleeds system and a purification system. Schematic of a typical nuclear power plant of PHWR type is shown in Figure 1. The cylindrical building, shown on left, houses the reactor and other equipment to produce steam which is delivered to the turbine, shown outside this building. The equipment in the steam circuit, also referred to as secondary circuit, are similar to those of a thermal power plant. The difference between the two is that, in the case of a nuclear power plant, the steam produced is not superheated and hence the steam cycle equipment are larger in size. This article briefly deals with the technological aspects of the nuclear equipment mainly located inside the cylindrical building, including the building itself which is known as reactor building or containment building.

Technological challenges in the design of a PHWR
Technological challenges posed in the design and construction of a nuclear power plant are unique in nature. It is a matter of great pride that this technology has been completely mastered by us in India. This article attempts to highlight, in a very brief manner, the summits we have conquered in the following, among many other, fields.
- Materials technology
- Design, theoretical and computational expertise
- Manufacturing technology
- Indigenisation- Project management

Materials technology
A PHWR needs, in addition to the commercial and conventional industrial materials, many other special materials and alloys manufactured to stringent specifications. For example, in the basic fuel material, which is required in an exceptionally pure form, the neutron absorbing impurities such as Boron, Cadmium, Dysprosium, Gadolinium, etc are controlled to very low ppm levels. Similar is the case for Zirconium alloys used in pressure tube, calandria tube, and fuel cladding. Likewise, the purity and the chemistry of heavy water is also required to be rigidly controlled. Even conventional materials used in the reactor core and associated systems - like stainless steel or carbon steel - have to meet special requirements, for example very low Cobalt impurity. Processing of materials like Uranium, Zirconium and heavy water has been fully developed in India and the technology has been translated to the production plants which are successfully operating as various units of the Development of Atomic Energy. The point to be highlighted here is that for successful design and operation of a NPP, wide variety of high purity materials and alloys needs to be developed not only at the laboratory scale but also on a regular production basis. (Indeed, establishment of a very strong industrial infrastructure in metallurgical and chemical engineering is one of the major challenges to be mastered by any country (particularly a developing country) wishing to embark on self-sufficient long term nuclear power programme). Even the concrete mix used for the reactor building and its internal structures are of special formulations such as heavy concrete, high performance concrete (M60 grade), etc.